Engine cylinders may be configured with one or more intake ports for receiving an aircharge. The one or more intake ports may diverge from a common intake passage or may have distinct intake ducts. Further, the one or more intake ports may receive fresh air or may recirculate exhaust gases.
One example of a cylinder with a split intake system is shown by Duret in U.S. Pat. No. 6,135,088. Therein, at low load conditions, a first inlet port of the cylinder is configured to receive recirculated exhaust gas while a second inlet port is configured to receive an air-fuel mixture that has been boosted following passage through a compressor.
However, the inventors herein have identified potential issues with such a system. As one example, addition of an exhaust turbine to drive the compressor may destroy the intended functionality of the system. Specifically, if a turbine were included in the common exhaust port, high pressure exhaust would be drawn into the lower pressure (that is, unboosted) inlet port leading to EGR control issues. As another example, it may be difficult to provide higher pressure EGR (HP-EGR) to further improve engine performance during some conditions.
Thus, in one example, some of the above issues may be at least partly addressed by a method of operating an engine comprising, providing a first aircharge at or below barometric pressure to an engine cylinder through a first intake passage and providing a second, boosted aircharge to the cylinder through a second, separate intake passage, the second aircharge boosted via an intake compressor driven by an exhaust turbine. In this way, an intake compressor may be driven by an exhaust turbine to provide a boosted intake aircharge to a cylinder through an intake passage while a naturally aspirated intake aircharge is provided to the cylinder, in parallel, through a separate intake passage. The different aircharges may then be mixed with each other, and fuel, in the cylinder prior to combustion.
In one example, the first intake passage may be coupled to a first exhaust passage while the second, separate intake passage is coupled to a second, separate exhaust passage. A turbocharger compressor coupled only to the second intake passage may be driven by a turbocharger turbine, coupled only to the second exhaust passage, to thereby provide a boosted aircharge in the second intake passage. In one example, fresh air at or below barometric pressure (BP) may be delivered to the cylinder via a first intake valve communicating with the first intake passage, while fresh air at compressor pressure (that is, boosted air) is delivered to the cylinder via a second intake valve communicating with the second intake passage. In this way, at least a portion of the fresh intake air can be provided via natural aspiration while the other portion is compressed. By reducing the portion of intake air requiring compression, compressor efficiency can be increased and control delays can be reduced. Additionally, the desired boost can be provided via a smaller compressor and turbine, without compromising boost performance.
In an alternate example, one or both of the first and second aircharges may include at least some fresh air and at least some recirculated exhaust gas. For example, at least some lower pressure EGR (LP-EGR), recirculated from the first exhaust passage to the first intake passage, may be mixed with fresh air at or below BP in the first intake passage to form a first aircharge. Additionally at least some higher pressure EGR (HP-EGR), recirculated from the second exhaust passage to the second intake passage, may be mixed with boosted fresh air in the second intake passage to form a second aircharge. The first and second aircharges may be mixed with each other, and directly injected fuel, in the cylinder prior to combustion. A timing of opening of distinct intake valves coupled to the distinct intake passages may be varied relative to each other so as to deliver the different aircharges at different points of an engine intake stroke. The intake valve timings may be further coordinated with the opening of distinct exhaust valves coupled to the distinct exhaust passages.
In this way, by flowing only a portion of the exhaust gas through the turbine while naturally aspirating the remaining portion of exhaust gas into the cylinder, heat recovered from the exhaust gas is increased and the thermodynamic efficiency of the turbine is improved. By naturally aspirating some fresh air into the engine cylinder while boosting the remaining portion, compressor work may be reduced, thereby improving compressor efficiency. Additionally, boost control delays can be reduced. By providing aircharge with varying ratios of recirculated exhaust gas and fresh air at different pressures to an engine cylinder via distinct intake passages, EGR benefits may be extended and turbocharger benefits can be improved, even with the use of a smaller turbocharger. As such, engine efficiency and performance can be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.